X-ray tube having controllable focal spot size

ABSTRACT

An X-ray tube having means for controlling the width and length of a focal spot area on the anode target surface, the width being adjustable by varying respective positive biasing voltages which are applied to slotted metal cups positioned between the filament and the anode target surface, and the length being externally controllable and continuously adjustable by positioning two movable metallic elements, each being maintained at zero potential or negatively biased with respect to the filament and disposed between a respective end portion of the filament and the slotted metal cups.

United States Patent Garewal et al.

[54] X-RAY TUBE HAVING CONTROLLABLE FOCAL SPOT SIZE [72] Inventors: Khem Garewal, Stamford, Conn.; Barry M. Singer, New York, NY.

[73] Assignee: The Maehlett Laboratories, Incorporated,

Springdale, Conn.

[22] Filed: May 18, 1970 [21] Appl.No.: 38,184

[52] US. Cl ..3l3/57 [51] Int. Cl ....H0lj 35/00 [58] Field ofSearch ..3l3/57,57X

[56] References Cited UNITED STATES PATENTS 2,011,540 8/1935 Lee ..313/57 51 Feb. 29, 1972 Beese ..3l3/57 Perry ..3l3/57 X Primary Examiner-Ronald L. Wibert Assistant ExaminerConrad Clark Attorney-Harold A. Murphy and Joseph D. Pannone [5 7] ABSTRACT An X-ray tube having means for controlling the width and length of a focal spot area on the anode target surface, the width being adjustable by varying respective positive biasing voltages which are applied to slotted metal cups positioned between the filament and the anode target surface, and the length being externally controllable and continuously adjustable by positioning two movable metallic elements, each being maintained at zero potential or negatively biased with respect to the filament and disposed between a respective end portion of the filament and the slotted metal cups.

15 Claims, 6 Drawing Figures X-RAY TUBE HAVING CONTROLLABLE FOCAL SPOT SIZE BACKGROUND OF THE INVENTION This invention relates, generally, to X-ray generating devices and is concerned, more particularly, with an X-ray tube having an adjustable focal spot size.

In conventional X-ray tubes, electrons are emitted thermionically from a filamentary cathode and are drawn electrostatically to a target surface on a spaced anode electrode. A high positive voltage, with respect to the cathode, is applied to the anode for the purpose of establishing, in the interelectrode space, a strong electrostatic field which accelerates the emitted electrons to very high velocities. These accelerated electrons, upon striking the target, expend their kinetic energy in the target material thereby generating X-rays.

In order to obtain maximum resolution the X-rays must appear to be emanating from a point source. However, the most practical point source obtainable in a conventional X-ray tube is in the form of a small square. Consequently, some types of prior art tubes are provided with line focusing means whereby the emitted electrons are concentrated in a flat beam which is focused onto a linear surface area of the target. Generally, the target surface is sloped at a predetermined angle with regard to the width dimension of the fiat electron beam, and an X-ray transparent window is disposed in a portion of the tube envelope in radial alignment with the target surface. Thus, from the direction of the X-ray transparent window, the linear focal spot area on the sloped surface of the target appears to be a small square, having side dimensions equal to the width of the linear focal spot area. Therefore, X-rays leaving the tube through the X-ray transparent window appear to be emanating from this small square area of the target, which commonly is referred to as the focal spot" of the tube.

For some purposes, such as irradiation, for example, obtaining a high intensity output may be more important than providing maximum resolution. Since X-ray intensity is dependent on the total power output of the tube, higher X-ray intensity is achieved by increasing the anode voltage or the electron emission current or both. However, the resulting increase in electron density, when the electron beam is focused on a small area of the target, may overheat and even vaporize the target material. Consequently, when high X-ray intensity is of prime importance, it is more advantageous to use a larger focal spot area than would be used if maximum resolution were the more important factor. Therefore, X-ray tubes, generally, are designed to provide a focal spot area suitable for the intended use of the tube. However, if an X-ray tube had an externally controllable and readily adjustable means for changing the size of the focal spot area as desired, it would be adaptable for use in various fields of X-ray technology where only specially designed tubes now are being used.

Generally, the aforementioned line focusing means may be adjusted to increase the width of the linear focal spot area on the target surface. Thus, the apparent focal spot can be transformed from a square to a rectangle having a height greater than its width. However, resolution of laterally extending objects varies inversely with the height of the apparent focal spot; and resolution of vertically extending objects varies inversely with the width. Therefore, in order to provide uniform resolution throughout the X-ray image, the apparent focal spot is advantageously maintained in the square configuration, even though the area may be increased. Thus, an X-ray tube having means for varying only the width of the focal spot area does not provide an adequate solution to the problem of changing the focal spot area to conform to the intended use of the X-ray tube.

SUMMARY OF THE INVENTION Accordingly, this invention provides an X-ray tube having means for independently adjusting the width and the length of the focal spot area on the target surface. The X-ray tube of this invention comprises an evacuated envelope having therein a filamentary cathode which is disposed in spaced relationship with an anode target surface. Two, concentrically spaced, metallic cups are disposed between the filament and the anode, each cup having centrally disposed in the base thereof a respective slot which is positioned in close space, aligned relationship with the filament. Two, bent metallic strips are disposed in insulating spaced relationship between the filament and the metallic cups, each strip having an end portion connected to externally controllable and readily adjustable means for moving the strip relative to a respective end portion of the filament, an intermediate portion disposed in space, parallel relationship with a respective end portion of the filament and an opposite end portion extending radially away from the filament in spaced, opposing relationship with the equivalent end portion of the other metallic strip. Positive biasing voltages are applied to the respective metallic cups and have maximum effect at the longitudinal edges of the respective slots whereby electrons emitted from the filament are focused into a flat beam which is incident on a linear surface area of the anode target surface. The respective metallic strips are maintained at zero or a negative potential with respect to the filament and have spaced opposing end portions positioned relative to one another by the externally controllable means. Electrons in the beam adjacent the spaced, opposing end portions of the metallic strips are repelled thereby reducing the width of the electron beam and, consequently, the length of the focal spot area on the target surface. However, even though the electron beam is reduced to a width less than the exposed length of the filament, full cathode current is obtained from the entire exposed length of the filament. Thus, the number of electrons bombarding the linear focal spot area on the target is higher than that obtained when the electron beam is mechanically stopped down to achieve a focal spot area of the same length.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of thisinvention, reference is made to the accompanying drawings wherein:

FIG. 1 is an axial view, partly in section, of an X-ray tube which embodies this invention;

FIG. 2 is a transverse, partly sectional, view taken along line 22 of FIG. 1 looking in the direction of the arrows and with the tube envelope omitted;

FIG. 3 is a fragmentary axial view, partly in section, taken along line 33 of FIG. 2 looking in the direction of the arrows;

FIG. 4 is a diagrammatic view illustrating the operation of this invention when the ears are in the fully open position;

FIG. 5 is another diagrammatic view illustrating the operation of this invention when the metal ears are in the fully closed position; and

FIG. 6 is a transverse, partly sectional, view similar to FIG. 2 showing another preferred embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring more particularly to the drawings wherein like characters of reference designate like parts throughout the several views, the X-ray tube shown in FIG. 1 comprises an evacuated, generally cylindrical envelope 10 of dielectric material, such as glass, for example. One end of envelope 10 is provided with a reduced diameter portion 12 which is integrally joined, by well-known methods, to one end of a coaxial reentrant portion 14. The other end of reentrant portion 14 is vacuum sealed, by conventional means, to one end of a metallic sleeve 16 made of kovar or the like. The opposite end of sleeve I6 is peripherally sealed, by standard techniques, to one end of a cylindrical block 18 of conductive material, such as copper, for example, which functions as the anode of the tube. The anode block 18 is provided with an axially extending, external stem 20 whereby the anode may be electrically connected to a positive voltage source. Accordingly, an interconnecting electrical lead 22 may be attached to the stem 20 by suitable means, such as machine screw 24 which threadingly engages the exposed end of stem 20, for example.

The anode block 18 extends axially within the envelope l and carries on the inner end thereof a target 26 of X-ray emitting material, such as tungsten, for example. The target 26 is inclined at an angle to the longitudinal axis of the tube for the purpose of directing X-rays, emanating from the target, out of the tube through an X-ray transparent window 28, such as glass, for example, which is vacuum sealed in the cylindrical wall of the tube.

The opposite end 32 of envelope is integrally joined to one end of a coaxial reentrant portion 34 which is similarly sealed, at its other end, to a stern press 36. Axially extending conductor posts 37-42 have respective portions thereof vacuum sealed, in spaced apart relationship, in the stem press 36 and exterior end portions which protrude out of the reentrant portion 34 for ease in making electrical connections to the conductor posts. As shown more clearly in FIG. 3, interior end portions of conductor posts 37 and 38 insulatingly support, by means of dielectric bushings 43 and 44, a metallic disc 46, known as the cathode head. The conductor posts extend insulatingly into a diametric channel 48, commonly referred to as the well," in the opposing surface of the cathode head 46. Disposed in the mouth of the well 48 in spaced, parallel relationship with the walls thereof, is a linear filament 50. The filament 50 may comprise a helically wound length of electron emitting material, such as tungsten, for example, and is fixedly attached, at the respective ends thereof, to the respective conductor posts 37 and 38.

interior end portions of the respective conductor posts 39 and 40 extend insulatingly, by means of dielectric bushings 52 and 54, respectively, through the cathode head 46 and into the well 48. The inner ends of the respective posts 39 and 40 are fixedly attached as by welding, for example, to respective ends of flexible wire leads 60 and 62, respectively. The wires 60 and 62 are provided with respective intermediate slack portions and each is fixedly attached at the opposite end to one end of a bent metallic strip 64 and 65, respectively. Adjacent portions of the respective metal strips 64 and 65 are secured, by conventional means, to adjacent end surfaces of radially extending, dielectric blocks 70 and 71, which may be made of ceramic material, for example, and are slidably disposed on shoulders 47 provided in the opposing sidewalls of the well 48. The metal strips 64 and 65 are provided with respective intermediate portions 66 and 67 which are positioned in close space, substantially parallel relationship with respective end portions of the filament 50. The other ends 68 and 69 of the respective metallic strips 64 and 65, are bent in the axial direction and extend radially away from the filament 50, thereby forming metallic ears" which are disposed in spaced, opposing relationship with one another.

The outer ends of the dielectric blocks 70 and 71 are affixed, by suitable means, to adjacent ends of respective radially extending shafts 72 and 73, each of which is fixedly attached at its opposite end to a respective colinear, larger diameter rod 74-75. The rods 74 and 75 extend axially within radially disposed bellows 76 and 77, respectively, and have opposite ends vacuum sealed to outer ends of the surrounding bellows. The inner ends of the bellows 76 and 77 are similarly sealed to raised central portions of respective kovar cups 81 and 82, which are peripherally sealed to adjacent ends of respective side reentrant portions 83 and 84 of the envelope 10, as shown in FIG. 1. The reentrant portions 83 and 84 extend radially outward and at their outer ends are integrally joined to radially extending portions 85 and 86, respectively, of the evacuated envelope 10.

The respective outer ends of the bellows 76 and 77 are pro vided with respective central apertures (not shown) which are aligned with respective, threaded cavities (not shown) in the attached ends of the rods 74 and 75. Metallic casings 87 and 88 enclose a respective bellows 76 and 77 and are each provided with internal, axially disposed shafts 89-90, which shafts are externally threaded and fixedly attached, at one end, to

the closed end of the respective casings. The shafts 89 and 90 threadingly engage respective aligned cavities in the adjacent ends of the respective rods 74 and 75; and the circular rims of the casings 87 and 88 seat in respective annular grooves sur' rounding the raised central portions in the kovar cups 81 and 82. The casings 87 and 88 may be conveniently rotated by turning knurled knobs 91 and 92 which are affixed, by conventional means, to the exposed ends of the casings.

When the casings 87 and 88 are rotated, the annular grooves in kovar cups 81 and 82 act as bearing surfaces for the circular rims of the respective casings, and consequently, the engaged ends of rods 74 and 75 travel inwardly or outwardly on the respective threaded shafts 89 and 90 depending on the direction of rotation. The bellows 76 and 77 expand or contract correspondingly with the movement of the attached end of the respective rods 74 and 75, thereby returning the ends of the bellows 76 and 77 in vacuum sealed relationship with the respective rods 74 and 75 and the evacuated envelope l0. Rotating the casings 87 and 88 in a clockwise direction draws the engaged ends of the respective rods 74 and 75 radially outward and expands the attached bellows 76 and 77. This movement of the engaged ends of rods 74 and 75 is transmitted through the attached shafts 72 and 73 to the slidable blocks 70 and 71, respectively. Guided by the confining walls of the well 48, the respective blocks 70 and 71 slide along the shoulders 47, thereby drawing the ears 68 and 69 of the respective metal strips 64 and 65 further apart and exposing a greater length of the filament 50. On the other hand, rotating the respective casings 87 and 88 in the counterclockwise direction results in the metal ears 68 and 69 of the respective metal strips 64 and 65 moving toward one another thereby reducing the exposed length of filament 50. The intermediate slack portions of the connected wires 60 and 62, respectively, provide the necessary additional wire required to permit movement of the respective metal strips 64 and 65 and still maintain respective electrical connections with the conductor posts 39 and 40. Thus, the respective metallic strips 64 and 65 are connected to externally controllable and readily adjustable means for moving the strips relative to one another and to respective adjacent end portions of the filament 50.

Spaced concentric cups 94 and 95, respectively, of metallic material such as molybdenum, for example, are disposed in spaced, enfolding relationship with the filament side of the cathode head 46. As shown in H6. 2, the base of each cup 94 and 95, respectively, is provided with a centrally disposed slot 96 and 97, respectively, which is slightly larger than the filament 50 and is aligned therewith. The metallic ears 68 and 69, respectively, extend insulatingly through the slot 96 which is longer than the slot 97 to permit radial movement of the ears 68 and 69. The rims of the respective cups 94 and are provided with diametrically aligned apertures 98-99 and 100-101, respectively, which permit the respective, smaller diameter shafts 72 and 73 and the dielectric blocks 70 and 71 to move insulatingly through the respective rims of the metallic cups 94 and 95, in a radial direction, as described previously. The respective casings 87 and 88 and enclosed bellows 76 and 77 are designed so that the outer ends of the bellows will contact the adjacent closed ends of the casings before the metallic ears 68 and 69 contact the respective adjacent ends of the slot 96. On the other hand, respective bellows 76 and 77 will be fully compressed before the metallic cars 68 and 69 contact one another by moving in the opposite radial direction. As shown in HO. 1, the metallic cups 94 and 95 are supported on the interior ends of conductor posts 42 and 41, respectively, which are fixedly attached to respective rims of the cups 94 and 95 by suitable means, as by welding, for example.

in operation, the filament 50 is connected, by means of conductor posts 37 and 38, respectively, to an external voltage source (not shown) which passes electrical current through the filament thereby heating it to incandescense and producing a copious emission of electrons therefrom. Zero or a negative biasing voltage with respect to the filament is applied, by

means of conductor posts 39 and 40, to the respectively connected, metal ears 64 and 65. A positive biasing voltage with respect to the filament is applied, by means of conductor posts 42 and 41, to the respectively attached metal cups 94 and 95. In some instances, the cup 95 may advantageously be maintained at a higher positive voltage potential than the cup 94.

The positive biasing voltages applied to the cups 94 and 95, respectively, have maximum effect at the edges of the respective slots 96 and 97 which are positioned in close space, surrounding relationship with the electron emitting filament 50. A very high positive voltage, in comparison to the positive voltages applied to the respective cups 94 and 95, is applied to the anode block 18 by means of electrical lead 22 and the stem 20.

When a conventional X-ray tube of the diode type is operated at relatively low anode voltages, full cathode current, ordinarily, cannot be achieved due to space charge buildup resulting from emitted electrons remaining in the vicinity of the filament. However, in X-ray tubes of the described type, the emitted electrons are drawn away from the filament by the electrostatic field established between the filament and the edges of the respective slots 96 and 97 in metallic cups 94 and 95. Because of the nonintercepting geometry of the slot edges and their close spaced relationship with the filament, the accelerated electrons pass through the slots 96 and 97, respectively, and into the stronger electrostatic field of the more positive anode 18. As a result, the electrons are accelerated to still higher velocities and bombard the target 26 with sufficient energy to generate X-rays which are directed out of the tube through the X-ray transparent window 28. Thus, the metallic cups 94 and 95 function as first and second accelerating grids, respectively, which provide means for achieving full cathode current independently of the anode operating voltage. Consequently, the anode 18 can be positioned a greater distance from the filamentary cathode 50 to provide more than adequate vacuum insulation therebetween and ensure that electrical breakdown will not occur at the highest anode voltages applied during normal operation of the tube.

When the metallic cups 94 and 95, respectively, are maintained at cathode potential and a relatively high positive voltage is applied to the anode 18, the electrons are drawn in a flat beam through the slots 96 and 97 and toward the target 26. However, just beyond the slot 97, the thickness of the electron beam tapers to a line where the electrons in the beam cross and follow diverging paths on the other side. Consequently, the electron beam increases in thickness as it approaches the anode 18 and, therefore, is incident on a relatively large, rectangular area of the target. When positive voltages are applied to the respective metallic cups 94 and 95, the electrons in the beam are deflected toward the longitudinal edges of the respective slots 96 and 97. As a result, the crossover line of the electron beam occurs a greater distance beyond the slot 97 and closer to the anode 18. Thus, by gradually increasing the positive voltages applied to the respective cups 94 and 95, the crossover line will be moved toward the anode 18, until it occurs on a linear surface area of the target, which may be referred to as the focal spot area. Thus, the metallic cups 94 and 95 not only provide means for achieving full cathode current at low anode operating voltages but also function as an electron lens system whereby the electron beam is brought into focus on a linear surface area of the target. Furthermore, by continuing to increase the positive voltages applied to the respective cups 94 and 95, the crossover line can be made to occur beyond the target thereby increasing the width of the focal spot area on the target surface. Thus, the externally controlled, positive voltages applied to the metallic cups 94 and 95 can be readily adjusted to obtain a focal spot area of the desired width.

Referring to FIG. 4, the ears 68 and 69 of the bent metallic strips 64 and 65, respectively, may be drawn as far apart as possible by rotating the casings 87 and 88 clockwise, as previously described. With the cars 68 and 69 in the fully open position, the entire length (L) of the filament is exposed to the target 26. The zero potential or negatively biased ears 68 and 69 repel electrons emitted from the adjacent ends of filament 50 thereby focusing the electron beam down to a width (L,) at the target. Consequently, the length of the linear focal spot area on the target surface is less than the exposed length (L) of the filament. However, electrons emitted from the entire length (L) of the filament contribute to the electron beam, thereby achieving full cathode current even though the width of the electron beam is reduced by an amount equal to (lo-L Since the surface of the target is inclined at an angle (0) with the longitudinal axis of the .beam, the resulting X-rays which leave the tube by way of the X-ray transparent window 28 appear to be emanating from a focal spot having a width equal to (L, sin 0).

Referring to FIG. 5, the ears 68 and 69 may be moved toward one another, by rotating the casings 87 and 88 oounterclockwise, as previously described. With the ears 68 and 69 positioned in closer space relationship with one another, the intermediate portions 66 and 67 of the metallic strips 64 and 65 are interposed between respective end portions of the filament 50 and the target 26. Thus, the exposed portion of the filament is reduced to a length (W). Furthermore, electrons are not emitted from the shielded end portions of the filament 50, because the respective opposing, intermediate portions 66 and 67 of the metallic strips are maintained at zero or a negative potential with respect to the filament. For similar reasons, electrons emitted from the exposed portions of the filament adjacent the respective ears 68 and 69 are repelled, thereby focusing the electron beam down to a width (W,) at the target. As a result, the length of the linear focal spot area on the target surface is less than the exposed length (W) of the filament. However, the electron beam is made up of electrons emitted from the entire exposed length (W) of the filament. Consequently, the resulting electron current is higher than would be achieved when the filament is mechanically stopped down" to obtain a focal spot area having a length equal to W at the target surface. The resulting X-rays leaving the tube through the X-ray transparent window appear to emanate from a focal spot having a width equal to (W, sin 0), which is greatly reduced in comparison to the width (L sin 0) obtained when the ears 68 and 69 are in the fully open position.

Thus, the externally controlled ears 68 and 69 may be readily adjusted relative to one another and to respective adjacent end portions of the filament 50 to obtain a focal spot area of the desired length and, consequently, an apparent focal spot of the desired width. Simultaneously, the externally controlled, positive bias voltages applied to the respective metallic cups 94 and 95 may be readily adjusted to obtain a focal spot area of the desired width and, as a result, an apparent focal spot of the desired height. In this manner, the focal spot area on the target surface may be modified to conform to the intended use of the X-ray tube, while the focal spot is maintained in a square configuration to provide uniform resolution. Thus, when practicing this invention, a single X-ray tube can be adapted to conform to the specific requirements in various fields of X-ray technology.

In FIG. 6, there is disclosed an alternative means for positioning the cars 68 and 69 relative to one another and to respective adjacent end portions of the filament 50. The outer ends of the dielectric blocks 70 and 71 are fixedly attached, by conventional means, to adjacent end portions of respective metal straps 111 and 112 which, preferably, are made of nonmagnetic material, such as kovar, for example. The opposing end portions of the respective straps 111 and 112 have respective outer surfaces attached, as by welding, for example, to adjacent ends of radially extending, helical springs 113 and 114, respectively, which may be made of stainless steel, for example. The outer ends of springs 113 and 114 are suitably attached to radially projecting ends of respectively aligned kovar pins 115 and 116 which have opposing end portions vacuum sealed in the cylindrical wall of envelope 10. Thus, the resilient forces exerted by the springs 113 and 114 on the attached straps 111 and 112 maintain the connected ears 68 and 69 in a normally open position with the entire length of the filament 50 exposed to the target 26. Open ended slots 127-128 and 129-130 are diametrically disposed in the respective cups 94 and 95 to allow the straps 111-112 and attached dielectric blocks 70-71 to pass insulatingly through the rims of the cups 94 and 95.

The end portions of straps 111 and 112 which are attached to springs 113 and 114 have opposite surfaces fixedly secured to adjacent end surfaces of cylinders 117 and 118, respectively. The cylinders 117 and 118 are made of magnetically permeable material, such as steel, for example, and are slidably disposed in respective U-shaped brackets or guides 119 and 120. The guides 119 and 120 are made of nonmagnetic material, such as molybdenum, for example, and have upstanding flanged portions fixed to diametrically opposing portions of the cathode head periphery. Thus, the guides 119 and 120 are suspended from the rim of the cathode head 46 in diametrically aligned relationship with one another and with a centrally disposed, electromagnetic coil 122. The electromagnetic coil 122 which is coated with an insulating material, such as epoxy resin, for example, is suspended from the exterior flat surface of the cathode head 46 by two spaced clamps 123 and 124 which may be made of nickel, for example. The coil 122 is provided with a centrally disposed core 121 of magnetically penneable material, such as a soft iron rod, for example, which extends longitudinally through the coil 122 and has opposing end surfaces disposed in spaced opposing relationship with the distal end surfaces of slidable cylinders 1 17 and 11 18. Respective opposite ends of the wire wound coil 122 are attached to electrical leads 125 and 126, respectively, which extend insulatingly through the stem press 20, in a well-known manner, to connect to an external voltage source (not shown).

In operation, the externally controlled voltage source connected to the coil 122 may be readily adjusted to pass a predetermined value of electrical current through the coil 122, thereby establishing a magnetic field of the desired intensity around the coil. As a result, the core 121 is magnetized and attracts the spaced, opposing end surfaces of the slidable cylinders 117 and 118. However, the attraction force exerted by the core 121 exceeds the restraining forces exerted by the respective springs 113 and 114 only by an amount sufficient to draw the respective cylinders l 17 and 118 radially inward the desired distance. An increase in electrical current through the coil establishes a stronger magnetic field which further exceeds the restraining forces of the respective springs 113 and 114, thereby drawing the respective cylinders 117 and 118 radially inward a greater distance. As a result, the dielectric blocks 70 and 71 slide transversely in the well 48 and the cars 68 and 69 are positioned in accordance with the value of electrical current passing through the coil 122. When the coil 122 is deenergized, the magnetic field established by the electrical current is reduced to near zero intensity and the springs 113 and 114 draw the cars 68 and 69 radially outward to the normally open position. Thus, this alternative means of adjusting the positional relationship of the ears 68 and 69 permits simple and efficient adjustment, while the X-ray tube is in operation, from an external voltage source which may be located at a remote distance from the tube. As a further alternative the electrical coil 122 may be replaced by two colinearly spaced coils, each controlling the movement of one of the respective cylinder 117 and 118, thereby providing independent means for positioning each ear 68, and 69, respectively.

Thus, there has been disclosed herein a novel X-ray tube having independent means for adjusting the width and the length of the focal spot area on the target surface. With a zero or negative potential applied to the respective cars 68 and 69, the length of the focal spot area may be varied by only slight linear displacements of the respective cars 68 and 69 thereby providing very sensitive control over the width of the resulting apparent focal spot. By varying the positive bias voltages ap plied to the respective cups 94 and 95, the width of the focal spot area may be adjusted independently of the respective ears 68 and 69, thereby providing very sensitive control over the height of the resulting apparent focal spot. in this manner, the apparent focal spot may be maintained in the square configuration while the linear focal spot area on the target surface is being adjusted to a size more suitable for the intended use of the Xray tube.

From the foregoing, it will be apparent that all of the objectives of this invention have been achieved by the structures shown and described herein. It will be also apparent, however, that various changes may be made by those skilled in the art without departing from the spirit of the invention as expressed in the appended claims. It is to be understood, therefore, that all matter shown and described is to be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An X-ray tube comprising:

an evacuated envelope;

an anode having a target portion within the envelope;

a cathode within the envelope in spaced relationship with the anode;

first means for directing electrons from the cathode onto an elongated area of the target;

movable control means positioned adjacent the cathode for adjusting the length of said elongated area independently of the width; and

means for moving said control means parallel to the cathode.

2. An X-ray tube comprising:

an evacuated envelope;

an anode supported by the envelope and having a target located within the envelope;

a cathode supported within the envelope and disposed in spaced relationship with the target;

grid means for directing electrons in a beam from the cathode to the anode and for focusing the beam onto a linear focal spot area of the target; and

movable control means for adjusting the length of the focal spot area independently of the width.

3. An X-ray tube as set forth in claim 2 wherein said grid means includes at least one conductive member disposed between the cathode and the anode in spaced apart relationship with said cathode and anode and means for rendering electron flow density from the cathode to the anode independent of the spacing between the cathode and the anode.

4. An X-ray tube as set forth in claim 3 wherein said means includes means for maintaining the conductive member at a preselected electrical potential with respect to the cathode.

5. An X-ray tube comprising:

an evacuated envelope;

an anode within the envelope and including a target;

a cathode within the envelope in spaced relationship with the anode and including a filament positioned in opposing relationship with the target;

grid means for directing electrons in a beam from the filament to theanode and for focusing them on a linear focal spot area of the target; and

movable control means for adjusting the length of the focal spot area independently of the width.

6. An X-ray tube as set forth in claim 5 wherein said grid means for focusing comprises at least one metallic member insulatingly disposed between the cathode and the anode, said metallic member having an aperture therein larger than the filament and aligned therewith, and means for maintaining said metallic member at an electrostatic potential with respect to the filament.

7. An X-ray tube as set forth in claim 6 wherein the metallic member is maintained at a positive potential with respect to the filament.

8. An X-ray tube as set forth in claim 5 wherein said movable control means includes a pair of conductive members disposed between the cathode and the grid means, each member having a portion thereof positioned closely adjacent a respective portion of the filament, and means for maintaining said members at zero or a negative potential with respect to the filament.

9. An X-ray tube as set forth in claim 8 wherein said portions of the members are movable with respect to said portions of the filament, and said control means includes means for moving said portions of the members relative to the filament.

10. An X-ray tube as set forth in claim 8 wherein said portions of the members are disposed longitudinally between the filament and the grid means and in spaced opposing relationship with one another, and said control means includes means for moving said portions of the members toward and away from one another.

11. An X-ray tube as set forth in claim 10 wherein said moving means is externally controllable 12. An X-ray tube as set forth in claim 11 wherein said envelope includes opposing wall portions having respective expansible devices vacuum sealed therein, and said externally controllable means includes two mechanical linkage systems, each having a portion thereof attached to a respective conductive member and another portion thereof connected to a respective expansible device.

13. An X-ray tube as set forth in claim 12 wherein the expansible devices each comprises a cylindrical bellows having one end vacuum sealed to a surrounding portion of the en velope wall and an opposite end hermetically sealed to said connecting portion of the respective mechanical linkage system.

14. An X-ray tube as set forth in claim 13 wherein said opposite ends of the bellows have internally threaded cavities and each bellows is enclosed by a metallic casing having an externally threaded shaft therein, said shaft threadingly engaging said cavity in the bellows whereby rotation of the casing causes expansion and contraction of the bellows thereby activating the connecting linkage to move the respective elongated members relative to one another and to the filament.

15. An X-ray tube as set forth in claim 11 wherein said externally controllable means comprises a pair of magnetically permeable members slidably disposed in spaced relationship with one another on the side of the cathode opposite the anode, each member being attached to a respective conductive member, resilient means for urging said members apart, and electromagnetic means disposed between said members for urging them toward one another. 

1. An X-ray tube comprising: an evacuated envelope; an anode having a target portion within the envelope; a cathode within the envelope in spaced relationship with the anode; first means for directing electrons from the cathode onto an elongated area of the target; movable control means positioned adjacent the cathode for adjusting the length of said elongated area independently of the width; and means for moving said control means parallel to the cathode.
 2. An X-ray tube comprising: an evacuated envelope; an anode supported by the envelope and having a target located within the envelope; a cathode supported within the envelope and disposed in spaced relationship with the target; grid means for directing electrons in a beam from the cathode to the anode and for focusing the beam onto a linear focal spot area of the target; and movable control means for adjusting the length of the focal spot area independently of the width.
 3. An X-ray tube as set forth in claim 2 wherein said grid means includes at least one conductive member disposed between the cathode and the anode in spaced apart relationship with said cathode and anode and means for rendering electron flow density from the cathode to the anode independent of the spacing between the cathode and the anode.
 4. An X-ray tube as set forth in claim 3 wherein said means includes means for maintaining the conductive member at a preselected electrical potential with respect to the cathode.
 5. An X-ray tube comprising: an evacuated envelope; an anode within the envelope and including a target; a cathode within the envelope in spaced relationship with the anode and including a filament positioned in opposing relationship with the target; grid means for directing electrons in a beam from the filament to the anode and for focusing them on a linear focal spot area of the target; and movable control means for adjusting the length of the focal spot area independently of the width.
 6. An X-ray tube as set forth in claim 5 wherein said grid means for focusing comprises at least one metallic member insulatingly disposed between the cathode and the anode, said metallic member having an aperture therein larger than the filament and aligned therewith, and means for maintaining said metallic member at an electrostatic potential with respect to the filament.
 7. An X-ray tube as set forth in claim 6 wherein the metallic member is maintained at a positive potential with respect to the filament.
 8. An X-ray tube as set forth in claim 5 wherein said movable control means includes a pair of conductive members disposed between the cathode and the grid means, each member having a portion thereof positioned closely adjacent a respective portion of the filament, and means for maintaining said members at zero or a negative potential with respect to the filament.
 9. An X-ray tube as set forth in claim 8 wherein said portions of the members are movable with respect to said portions of the filament, and said control means includes means for moving said portions of the members relative to the filament.
 10. An X-ray tube as set forth in claim 8 wherein said portions of the members are disposed longitudinally between the filament and the grid means and in spaced opposing relationship with one another, and said control means includes means for moving said portions of the members toward and away from one another.
 11. An X-ray tube as set forth in claim 10 wherein said moving means is externally controllable.
 12. An X-ray tube as set forth in claim 11 wherein said envelope includes opposing wall portions having respective expansible devices vacuum sealed therein, and said externally controllable means includes two mechanical linkage systems, each having a portion thereof attached to a respective conductive member and another portion thereof connected to a respective expansibLe device.
 13. An X-ray tube as set forth in claim 12 wherein the expansible devices each comprises a cylindrical bellows having one end vacuum sealed to a surrounding portion of the envelope wall and an opposite end hermetically sealed to said connecting portion of the respective mechanical linkage system.
 14. An X-ray tube as set forth in claim 13 wherein said opposite ends of the bellows have internally threaded cavities and each bellows is enclosed by a metallic casing having an externally threaded shaft therein, said shaft threadingly engaging said cavity in the bellows whereby rotation of the casing causes expansion and contraction of the bellows thereby activating the connecting linkage to move the respective elongated members relative to one another and to the filament.
 15. An X-ray tube as set forth in claim 11 wherein said externally controllable means comprises a pair of magnetically permeable members slidably disposed in spaced relationship with one another on the side of the cathode opposite the anode, each member being attached to a respective conductive member, resilient means for urging said members apart, and electromagnetic means disposed between said members for urging them toward one another. 